Turns out that being short of time is not longer an excuse for not exercising.

New research finds that just a few minutes of intense activity interspersed between less intense stretches of exercise will burn excess calories all day long.

Men participating in a small study burned an additional 200 calories a day by doing a workout for less than 25 minutes that included a few minutes of hard, intense exercise on a stationary bike, spaced between less intense activity.

The technique, called sprint-interval training, is used by athletes to improve performance. It seems like a reasonable strategy for weight maintenance — and for those days when you just can't get in a full workout, the researchers said.

"The harder you work, the more calories you will burn per minute," said study leader Kyle Sevits, a researcher at Colorado State University in Fort Collins.

Sevits looked at five healthy men of normal weight with an average age 28. They were asked to ride an exercise bike as hard as they could five times for 30 seconds each. In between, they did resistance-free pedaling for four minutes. In all, the workout took less than 25 minutes.

Sevits compared the men's energy output on two days, including the day of the cycling exercise. The extra calories were burned over 24 hours on the workout day, despite the brief time spent in hard exercise.

The findings were presented last week at an exercise conference in Colorado sponsored by the American Physiological Society, the American College of Sports Medicine, and the Canadian Society for Exercise Physiology.

A British expert finds the results intriguing.

"This study provides some interesting preliminary data showing that sprint-interval training can increase 24-hour energy expenditure," said Dr. Stuart Gray, a lecturer in exercise physiology at the University of Aberdeen in Scotland. Gray was not involved in the study.

The participants didn't appear to consume more food than normal or reduce other daily activities to compensate for the bursts of activity, he said.

However, "this kind of exercise is very intense and may not suit all," he cautioned. "My advice would be to not simply focus on one form of exercise but try to increase activity and reduce consumption wherever and whenever you can."

Still, the sprint technique ''is a useful option if time is a limiting factor," he said.

Sevits focused on the practical aspects of the research. "Many people gain a couple of pounds a year," he said, adding that demanding work schedules and family duties can limit exercise time. "A time-sensitive exercise that will help them burn some calories and keep them fit may help ward off those extra couple of pounds."

Sevits can't say the technique would work for weight loss, as that was not studied.

Would another form of exercise burn calories equally well?

"If people do not have access to a gym, they can try to find a grassy hill," Sevits said. "Sprint as fast as possible up it for 30 seconds, then walk back down."

A treadmill would not work well for sprint-interval training, as it goes at a constant speed, he said.

Because the research findings were not yet published in a peer-reviewed journal, they should be considered preliminary.